The invention relates to an inductive flow meter wherein a transversely directed magnetic field is generated within a tube, the magnetic field, together with an electrically conductive liquid flowing in the tube, generating a voltage which is detected by electrodes arranged at said tube and is evaluated. Such an inductive flow meter is known, among others, from US Pat. No. 5,018,391 to Doll.
In the above mentioned known flow meter, positive and negative exciting currents are generated alternatingly in the magnetic field generator, the currents in turn generating a magnetic flux density B within the tube. The exciting current flows for less than one cycle of the applied alternating voltage, respectively. In an evaluating circuit connected to the electrodes, the integral with respect to the respective cycle of the alternating voltage is formed. Since the forming of the integral is performed for exactly one cycle, parasitic voltages coupled in from the mains and having the same frequency as the exciting voltage are compensated. The value of the integral at the end of the mains cycle is supplied to a differential amplifier supplying the useful signal which may afterwards be freed from further parasitic signals. The principle of the known flow meter is based on the fact that the magnetic flow density B.sub.2 at the end of the cycle used for forming the integral is equal to the magnetic flow density B.sub.1 at the beginning of the excitation. However, in practice, this condition is met only approximately. While the exciting current is zero at the beginning and the end of the integration time, the flux density B is not. This is due to the retentivity inherent to the iron of the magnetic field generator. This retentivity has the effect that, at the beginning of the integration interval, the flux density still has a value derived from the previous exciting pulse of opposite polarity and falsifying the measuring result, since the integral of the inductive parasitic voltage ##EQU1## will be eliminated only if B.sub.2 =B.sub.1.